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hyundai excel overdrive problem

I have just bought a 1993 Hyundai Excel from an old lady with 54,000km on it in mint condition. The problem that I am having is when you go down a hill and take your foot of the gas the car seems like the brake is going on and off slightly. I thought it might need a tune up in which I did and it never helped. Going along the road the car is fine(not 100% smooth but fine)I have narrowed the problem down to the overdrive, when it is turned off the car is perfect going down hill and all. The car has a button to turn the overdrive on and off, so I am wondering is it electrical problem causing maybe the overdrive not to work correctly or is in the transmission?